After three decades of hopes tempered by setbacks, gene therapy — the process of treating a disease by modifying a person's DNA — is no longer the future of medicine, but is part of the present-day clinical treatment toolkit.

Here we look at some of the key developments that have led to several successful gene therapy treatments for patients with serious medical conditions, as well as discuss emerging genome editing techniques such as CRISPR/Cas9, which provides a way to correct or alter an individual’s genome with precision, that lead to more effective gene therapy approaches.

Gene therapy is designed to introduce genetic material into cells to compensate for or correct abnormal genes. If a mutated gene causes damage to or spurs the disappearance of a necessary protein, for example, gene therapy may be able to introduce a normal copy of the gene to restore the function of that protein.

Approaches that have delivered the best outcomes in gene therapy include: The direct in vivo administration of viral vectors, or the use of viruses to deliver the therapeutic genes into human cells. And, the transfer of genetically engineered blood or bone marrow stem cells from a patient, modified in a lab, then injected back into the same patient.

Originally envisioned as a treatment solely for inherited disorders, gene therapy is now being applied to acquired conditions such as cancer. For example, the engineering of lymphocytes, white blood cells, that can be used in the targeted killing of cancer cells.

In 2017, a steady stream of encouraging clinical results showed progress in gene therapies for hemophilia, sickle-cell disease, blindness, several serious inherited neurodegenerative disorders, an array of other genetic diseases, and multiple cancers of the bone marrow and lymph nodes.

Three gene therapies have been approved by the US Food and Drug Administration in the past year, and many more are under active clinical investigation